The invention relates to a metabolic support system including a solution, method and apparatus for sustaining organs for transplantation under near-physiologic conditions. More particularly, the invention relates to use of the system for repair and/or long-term maintenance of organs for transplantation, as a pharmaceutical delivery system and prognosticator of posttransplantation organ function.
There continues to be an extreme shortage of organs for transplantation. Currently, the major limiting factor in clinical transplantation is the persistent shortage of organs. For example, kidney transplantation is largely dependent upon the availability of organs retrieved from heart-beating cadaver donors. There exists, however, a large and as yet untapped source of organs for transplantation, namely, non-heart-beating cadavers. Non-heart-beating cadavers are accident victims who succumb at the site of an injury and those having short post-trauma survival times. Additionally, non-heart-beating cadavers result when families are emotionally unable to make the decision to donate the organs of a loved one contemporaneously with making the decision to terminate life support. In these situations, the organs are not used because the lack of circulating blood supply (warm ischemia) once the heart stops beating, results in an injury cascade.
An organ marginally, but functionally damaged by warm ischemia cannot tolerate further damage mediated by the hypothermic conditions presently utilized to preserve organs intended for transplantation. Under these conditions, the lipid bilayer experiences a phase-change and becomes gel-like, with greatly reduced fluidity. The essentially frozen lipid in the cell membranes negates the utilization of O2-tension. The metabolic consequence is glycolysis, which is analogous to the state of anoxia. It has been described that below 18xc2x0 C., hypothermia inhibits the tubular and glomerular activities of the kidney and that at 4xc2x0 C., the utilization of oxygen is approximately 5% of that at normothermia.
Hypothermic storage can also produce vasospasm and subsequent edema in an organ. Hypothermically preserved organs can experience glomerular endothelial cell swelling and loss of vascular integrity along with tubular necrosis; phenomenon attributable to the hypothermic conditions employed. Hypothermia can also inhibit the Na/K dependent ATPase and result in the loss of the cell volume regulating capacity. The loss of volume regulation is what causes the cellular swelling and damage. An ample supply of oxygen does not actively diminish the amount of this swelling because the cell membrane is essentially frozen, preventing the effective utilization of oxygen. Without adequate oxygen delivery, the anoxia leads to disintegration of the smaller vessels after several hours of perfusion. The lack of oxygen and the subsequent depletion of ATP stores mean that anaerobic glycolysis is the principal source of energy under traditional preservation conditions. The subsequent loss of nucleosides is probably a very important factor in the failure of tissues subjected to warm ischemia and prolonged periods of cold ischemia to regenerate ATP after restoration of the blood supply. The inability to supply adequate oxygen has led to the routine reliance on hypothermia for organ preservation. In the case of warm ischemia, circulatory arrest leads to anoxia where there is no molecular oxygen for oxidative phosphorylation. The lack of molecular oxygen leads to the accumulation of NADH and the depletion of ATP stores with in the mitochondria.
Thus, ischemia (whether warm ischemia or cold ischemia) is an injury cascade of events that can be characterized as a prelethal phase, and a lethal phase. The prelethal phase produces harmful effects in three ways: hypoxia; malnutrition; and failure to remove toxic metabolic wastes. With the lack of circulating blood comes a lack of molecular oxygen. The resulting hypoxia induces depletion of energy stores such as the depletion of ATP stores in mitochondria. Depletion of ATP leads to cellular changes including edema, loss of normal cellular integrity, and loss of membrane polarity. The cellular changes, induces the lethal phase of ischemia resulting in accumulation of metabolic wastes, activation of proteases, and cell death.
The perfusate solution that represents the current state-of-the-art in hypothermic organ preservation, and provides for optimized organ preservation under hypothermic conditions, contains components which prevent hypothermic induced tissue edema; metabolites which facilitate organ function upon transplantation; anti-oxidants; membrane stabilizers; colloids; ions; and salts (Southard et al., 1990, Transpl. 49:251; and Southhard, 1989, Transpl. Proc. 21:1195. The formulation of this perfusate is designed to preserve the organs by hypothermic induced depression of metabolism. While it minimizes the edema and vasospasm normally encountered during hypothermic storage, it does not provide for the utilization of a substantially expanded donor pool.
This is due to the fact that an organ or tissue damaged by warm ischemia cannot tolerate further damage mediated by the hypothermia. Even with just 30 minutes of ischemic, the postransplant function of an organ can be compromised. For example, using organs from heart beating cadavers (non-damaged), the immediate nonfunction rate is estimated to be 25%; and within just 30 minutes of warm ischemia, the immediate nonfunction rate is increased to about 60%. Thus, 60% of the kidneys from non-heart-beating cadavers do not immediately function because of prelethal ishchemic injury. Further, irreversible ischemic damage and injury is thought to occur to organs deprived of blood flow in just a few hours or less (Klatz et al., U.S. Pat. No. 5,395,314). Unless new sources of organs can be developed, the number of transplantation procedures will remain constant. Additionally, the donor pool cannot be substantially expanded because there is no process/system available to repair prelethal ischemic damage in warm ischemically damaged organs or tissues.
Recent efforts have focused on prevention of ischem ic damage by intervening with a solution immediately upon cessation of blood flow. For example, a protective solution, disclosed in U.S. Pat. No. 4,415,556, is used during surgical techniques or for organs to be transplanted for preventing ischemic damage to the organ. The protective solution is used as a perfusate to improve aerobic metabolism during the perfusion of the organ. U.S. Pat. No. 5,395,314 describes a method of resuscitating a brain by circulating, after interruption of the blood supply, through the brain a hypothermic preservation solution (approximately 8-10xc2x0 C.) designed to lower organ metabolism, deliver oxygen, and inhibit free radical damage.
Although such methods and preservation solutions are useful in preventing ischemic damage in organs, these beneficial effects are overshadowed by practical and functional limitations. First, for such methods and solutions to be effective in preventing ischemic damage, they must be applied immediately (within minutes) after interruption of the blood supply. Logistic restraints, as in the case where an accident victim becomes an organ donor, may severely curtail the use of such methods and solutions. For example, their use is impractical at the site of an accident or in the ambulance where initiation of the ischemic injury cascade would occur. Secondly, irreversible ischemic damage and injury is thought to occur to organs deprived of blood flow in minutes (e.g., brain) or within just a few hours (heart, kidney). An organ or tissue, marginally, but functionally, damaged by warm ischemia cannot tolerate further damage mediated by hypothermic storage prior to transplantation, or restoration of blood flow upon transplantation. One reason is that restoration of the circulation after ischemic-reperfusion may paradoxically result in further tissue damage. (McCord et al., 1985, N Engl J Med 312:159-163). During reperfusion, reoxygenation of ischemically damaged tissue can result in further tissue injury caused through the formation of oxygen free radicals, depletion of free radical scavengers, and the release of chemotactic agents.
Thus, there is a need for a system, including a preservation solution useful for initial organ flushing and as a perfusate for in situ or ex vivo preservation of organs for transplantation, which employs a warm preservation technology which minimizes, and, in fact, repairs damage due to warm ischemia, and which supports the organ near normal metabolic rate. Portability and automation of the system is important, particularly in situations where the system is used to initiate organ preservation in situ either prior to or immediately following termination of life support or at external sites following an accident where cardiac arrest has occurred.
In one aspect, the invention relates to an exsanguinous metabolic support system for maintaining an organ, tissue or section of anatomy in a near normal metabolic state outside of, or at least isolated from the circulatory system of the body. The system comprises an organ chamber for holding an organ, having means to collect organ product generated during perfusion; a perfusion delivery subsystem comprising one or more perfusion fluid paths for circulating and regenerating a warm perfusion solution capable of supporting the organ in a near normal metabolic state; a controlled gassing subsystem for regulation of respiratory gases and pH of the perfusate; a temperature controller for controlling temperature of the perfusate; and a monitoring subsystem for monitoring various parameters of the perfusate.
In a related aspect, the invention relates to a monitoring subsystem in which the monitoring of various parameters of the perfusion solution is computer controlled. Such a system would include a computer, and sensor means interposed in the perfusate flow path, and coupled to the computer for sensing at least one of the temperature, pH, pressure, flow rate, PaO2, PaCO2, and osmolarity of the perfusion solution and providing the sensed information to the computer.
In another aspect, the invention relates to a method for the maintenance of an organ or tissue for transplantation, comprising the steps of flushing the organ with a buffered physiological solution to remove blood and blood products, establishing and maintaining the organ in the exsanguinous metabolic support system of the present invention, and monitoring the functional integrity of the organ.
In yet another aspect, the invention relates to a method for the effective delivery of an exogenous molecule to a target tissue comprising the steps of removing a target tissue or organ from a living body, flushing the organ with a buffered physiological solution to remove blood and blood products having accumulated in the organ during isolation of the organ, maintaining the target tissue in a recirculating, oxygenated perfusion solution, in a near normal metabolic state at 25-37xc2x0 C.; contacting the target tissue with the exogenous molecule to be delivered in the perfusion solution, and then returning the organ or tissue to the body of the donor or another recipient. Where there has been any form of insult or injury which compromises the functional integrity of the organ, for example, where a period of more than 30 minutes since cessation of blood flow to the organ has occurred, it is desirable to monitor the function of the organ prior to exposure to the therapeutic agent to be delivered to evaluate the functional integrity of the organ and determine whether there has been irreparable damage to the tissue which will affect the ability of the organ to function normally posttransplantation.
In a related aspect, the invention relates to a method for the effective delivery of an exogenous molecule to a target tissue comprising the steps of isolating a target tissue or organ from the rest of the circulatory system of a living body, flushing the organ with a buffered physiological solution to remove blood and blood components, maintaining the target tissue in a recirculating, oxygenated perfusion solution, in a near normal metabolic state at 25-37xc2x0 C., contacting the target tissue with the exogenous molecule to be delivered in the perfusion solution of the invention for a period of time sufficient to effectuate delivery of the molecule and then returning the organ or tissue to the rest of the circulatory system of the body.
In yet another aspect, the invention relates to a solution to be in employed in an exsanguinous metabolic support system comprising a buffered basal medium which contains essential and non-essential amino acids, carbohydrates, metabolites, inorganic ions, serum proteins, lipids, hormones, nitrogen bases, vitamins, reducing agents and a buffering system and additional components for oxidative metabolism including coenzyme A, FAD, DPN, Cocarboxylase, TPN, 2xe2x80x2-deoxyadenosine, 2xe2x80x2deoxyguanosine, 2xe2x80x2-deoxycytidine, thymidine, adenosine, guanosine, cytidine, uridine, ATP, AMP, and UTP.
In still another aspect, the invention relates to a method for storing a tissue, explant or organ intended for transplantation comprising the steps of flushing the tissue, explant or organ with a non-blood buffered physiological solution to remove blood and blood products; perfusing the tissue, explant or organ in a warm preservation system capable of maintaining the tissue, explant or organ at a near normal rate of metabolism for a period of time sufficient to impart protection to the tissue, explant or organ; and storing the organ at 4-8xc2x0 C.